Published on Web 03/08/2008
Carboxylation of an ansa-Zirconocene Dinitrogen Complex: Regiospecific Hydrazine Synthesis from N2 and CO2 Donald J. Knobloch, Hannah E. Toomey, and Paul J. Chirik* Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell UniVersity, Ithaca, New York 14853 Received February 2, 2008; E-mail:
[email protected] Dinitrogen, N2, and carbon dioxide are two abundant and readily accessed atmospheric gases that could, in principle, be useful synthons for organic compounds.1 The kinetic and thermodynamic stability of both molecules presents significant challenges in designing efficient chemical transformations based on these two potential feedstocks. The discovery of early transition-metal dinitrogen compounds with four electron reduced, [N2]4- ligands2-4 offers the potential to form new nitrogen-carbon bonds using CO2 by both cycloaddition5 and insertion-type pathways.6 Here we describe the regioselective synthesis of N,N′-dicarboxylated hydrazines where the selectivity of the N-C bond forming reaction is controlled by a C2-symmetric ansa-zirconocene compound.7 Our laboratory has previously reported that the hafnocene dinitrogen complex, [(η5-C5Me4H)2Hf]2(µ2, η2, η2-N2) (1-N2), inserted 2 equiv of carbon dioxide to predominately yield 1-NN(CO2)2, where the same nitrogen atom was carboxylated twice (eq 1).8 A small amount (∼15%) of the N,N′-functionalized product was observed. Subsequent treatment of 1-NN(CO2)2 with excess Me3SiI liberated the silylated N,N-dicarboxylated hydrazine. Synthesis of the hafnium congener was inspired by the observation that 1-N2 has a higher barrier for deleterious side-on, end-on isomerization of the N2 ligand and is less prone to undergo dinitrogen loss.9 Here we report that such functionalization pathways are available to zirconium compounds if the appropriate ligand environment is present.
Based on the hafnium chemistry, our laboratory sought to prepare a zirconocene dinitrogen complex that is resistant to ligand-induced N2 loss12 and mimics the carboxylation chemistry of 1-N2. The configurational stability and well-established10 electronic properties of [Me2Si]-bridged ansa-zirconocenes made this class of compounds attractive for N2 carboxylation. We recently reported the diastereoselective synthesis of the dimeric ansa-zirconocene dinitrogen complex, [Me2Si(η5-C5Me4)(η5-C5H3-3-tBu)Zr]2(µ2, η2, η2-N2) (2N2).11 Both X-ray crystallography and multinuclear NMR studies established sole formation of the C2 symmetric, homochiral dimer. We were pleased to find that 2-N2 undergoes extremely slow exchange with 15N2 gas, establishing a high barrier for ligandinduced side-on, end-on isomerization.11 In fact, 2-15N2 is best prepared from Na(Hg) reduction of the zirconocene diiodide, 2-I2, in the presence of 15N2. Use of the zirconocene diiodide rather than the dichloride precursor eliminates trace contamination from zirconium(III) monohalide compounds that interfere with subsequent CO2 chemistry. 4248
9
J. AM. CHEM. SOC. 2008, 130, 4248-4249
Figure 1. Partial benzene-d6 13C NMR spectrum of 2-(N13CO2)2 (left) and 2-(15N13CO2)2 (right).
Addition of 2 equiv of CO2 to a green pentane solution of 2-N2 resulted in immediate formation of an orange precipitate identified as the CO2 insertion product, 2-(NCO2)2, where N2 carboxylation has occurred at each nitrogen atom (eq 2). Addition of 1 equiv of CO2 to 2-N2 resulted in 50% conversion to 2-(NCO2)2.
Special care must be exercised in handling 2-(NCO2)2 as decomposition to insoluble and unidentified products occurred over the course of hours (t1/2 ∼ 30 min) in benzene-d6 solution at 23 °C. Once isolated as a solid, 2-(NCO2)2 is stable for weeks in an inert atmosphere. Similar behavior was observed for the hafnocene complex, 1-NN(CO2)2, although solution decomposition occurred on the time scale of days under similar conditions.8 Although the instability of 2-(NCO2)2 has thus far precluded structural determination by X-ray diffraction, multinuclear NMR experiments, IR spectroscopy, combustion analysis, and reactivity studies definitively establish the regiochemistry of dinitrogen carboxylation. The benzene-d6 1H NMR spectrum of 2-(NCO2)2 exhibits the number of resonances expected for a C2 symmetric dimeric zirconocene. Likewise, a single 13C resonance was observed at 166.0 ppm, which splits into an AA′XX′ pattern upon labeling with 15N2 gas (Figure 1). Simulating the data yielded coupling constants similar in magnitude to those reported for 1-NN(CO2)2,8 confirming N2 carboxylation. 15N NMR spectroscopy also established functionalization of each nitrogen atom. A single resonance centered at 209.5 ppm was observed, comparable to the value of 184.4 ppm for the carboxylated nitrogen in 1-NN(CO2)2.8 If the structure of 2-(NCO2)2 was analogous to the hafnocene compound where one nitrogen atom 10.1021/ja8008506 CCC: $40.75 © 2008 American Chemical Society
COMMUNICATIONS
Figure 2. N,N′-Dicarboxylated hydrazine synthesis from treatment of 2-(NCO2)2 with various electrophiles.
was functionalized twice, C2 symmetry and single 13C and 15N NMR resonances would not be observed. Thus, the C2 symmetry imparted by the ansa-zirconocene ligand environment translates onto the regiochemistry of nitrogen carboxylation. The sterically demanding tert-butyl substituents likely direct the approach of the inserting CO2 molecule to the more open lateral positions of the metallocene wedge, although the opposite configuration, whereby the CO2 inserts syn to the tBu, has not been definitively eliminated. Treatment of a benzene-d6 solution of 2-(NCO2)2 with excess Me3SiI rapidly generated 2-I2 along with N,N′-dicarboxylated hydrazine, [(Me3Si)(Me3SiO2C)N]2 (3) (Figure 2). While the mass spectral data of 3 and its 13C and 15N isotopologues are identical to that previously reported for the N,N-isomer, the multinuclear NMR spectroscopic data are distinctly different and firmly establish its identity as the N,N′-dicarboxylated isomer.13 Motivated by these findings, the synthesis of other N,N′dicarboxylated hydrazines coupled to a second N-C bond forming reaction was targeted. Treatment of a benzene-d6 solution of 2-(NCO2)2 with 3 equiv of CH3OTf furnished a 1:1 ratio of the zirconocene bis(triflate), 2-(OTf)2, and a second zirconium compound, 4, with new nitrogen-carbon bonds. 2-(OTf)2 was prepared in a straightforward manner from 2-I2 and AgOTf. Preparation of various isotopologues of 4 with 15N2, 13CO2, 13CH3OTf, and CD3OTf along with protonation and 2-D NMR experiments13 establish the identity of the product as the ansa-zirconocene hydrazido where a total of three new N-C bonds (two from CO2, one from CH3OTf) have been assembled (Figure 2).14 Because we have been unable to obtain X-ray quality crystals of 4, the hapticity and the relative stereochemistry of the hydrazido ligand in the zirconocene wedge has not been established. Although excess methyl triflate is not a sufficiently potent electrophile to cleave to the Zr-N(hydrazido) bond in 4, free hydrazine, 5, was obtained upon hydrolysis (Figure 2). A parent ion peak of 162 amu was observed for the natural abundance hydrazine, and the appropriate isotopic perturbations were observed upon preparation of the d9 isotopologue from CD3OTf, 13C2 isotopologues from independent additions of 13CH3OTf and 13CO2, respectively, and the 13C4 isotopologue from the combination of 13CH OTf and 13CO . While excess MeOTf and MeI proved 3 2 ineffective for removing the free hydrazine (even with heating), addition of excess Me3SiI to 4 produced a mixture of the previously reported silylated hydrazine, 6,15 along with 2-I2 and Me3SiOTf (Figure 2).
In summary, a C2-symmetric ansa-zirconocene dinitrogen complex promotes selective insertion of 2 equiv of CO2 into Zr-N bonds resulting in the synthesis of a variety of N,N′-dicarboxylated hydrazines from two inert atmospheric gases, N2 and CO2. The functionalized product can be treated with silyl- and carbon-based electrophiles to form new N-Si and N-C bonds, respectively.14 These results open new synthetic pathways for the selective synthesis of organic molecules from abundant chemical feedstocks and demonstrate that such N2 functionalization with heterocumulenes reactions are available to zirconium. Acknowledgment. We thank the Director, Office of Basic Energy Sciences, Chemical Sciences Division, of the U.S. Department of Energy (DE-FG02-05ER15659) for financial support. P.J.C. is a Cottrell Scholar sponsored by the Research Corporation, a David and Lucile Packard Fellow in science and engineering, and a Camille Dreyfus Teacher-Scholar. Supporting Information Available: Complete ref 1 citation, experimental procedures, and select NMR spectroscopic data. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15)
Marks, T. J. et al. Chem. ReV. 2001, 101, 953. MacLachlan, E. A.; Fryzuk, M. D. Organometallics 2006, 25, 1530. Chirik, P. J. Dalton Trans. 2007, 16. Pool, J. A.; Bernskoetter, W. H.; Chirik, P. J. J. Am. Chem. Soc. 2004, 126, 14326. Guiducci, A. E.; Cowley, A. R.; Skinner, M. E. G.; Mountford, P. J. Chem. Soc., Dalton Trans. 2001, 1392. (a) Chisholm, M. H.; Extine, M. W. J. Am. Chem. Soc. 1977, 99, 792. (b) Andersen, R. A. Inorg. Chem. 1979, 18, 2928. For application of ansa-cyclopentadienyl ligands in stereospecific olefin polymerization, see: Coates, G. W. Chem. ReV. 2000, 100, 1223. Bernskoetter, W. H.; Lobkovsky, E.; Chirik, P. J. Angew. Chem., Int. Ed. 2007, 46, 2858. Bernskoetter, W. H.; Olmos, A. V.; Pool, J. A.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2006, 128, 10696. Zachmanoglou, C. E.; Docrat, A.; Bridgewater, B. M.; Parkin, G.; Brandow, C. G.; Bercaw, J. E.; Jardine, C. N.; Lyall, M.; Green, J. C.; Keister, J. B. J. Am. Chem. Soc. 2002, 124, 9525. Hanna, T. E.; Keresztes, I.; Lobkovsky, E.; Chirik, P. J. Inorg. Chem. 2007, 46, 1675. Bernskoetter, W. H.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2005, 127, 14051. See the Supporting Information for details. Hirotsu, M.; Fontaine, P. P.; Zavalij, P. Y.; Sita, L. R. J. Am. Chem. Soc. 2007, 129, 12690. Kalinin, A. V.; Apasov, E. T.; Bugaeva, S. V.; Ioffe, S. L.; Tartakovskii, V. A. Bull. Acad. Sci. USSR DiV. Chem. Sci. 1983, 32, 1282.
JA8008506
J. AM. CHEM. SOC.
9
VOL. 130, NO. 13, 2008 4249
